1. Field of the Invention
The present invention relates to an improvement of a fuel cell which is useful in an electric vehicle and the like.
2. Description of the Related Art
A fuel cell is a device in which a pair of electrodes are contacted with each other via an electrolyte, fuel is supplied to one of the electrodes, an oxidant is supplied to the other electrode, and the fuel in the fuel cell is electrochemically oxidized whereby chemical energy is directly converted into electrical energy. According to the kind of the electrolyte, fuel cells are classified into several kinds including a so-called solid polymer fuel cell in which a solid polymer electrolyte membrane is used as an electrolyte, and a phosphoric acid fuel cell in which phosphoric acid is used. Recently, a solid polymer fuel cell received attention as a fuel cell which can output high power. Therefore, a solid polymer fuel cell will be described as an example.
In a solid polymer fuel cell, when hydrogen gas is supplied to an anode, and oxygen gas is supplied to a cathode, and a current is output to an external circuit, the following reactions occur: EQU anode reaction: H.sub.2.fwdarw.2H++2e.sup.- (1)
cathode reaction: 2H++2e-+(1/2)O.sub.2.fwdarw.H.sub.2 O (2)
At this time, hydrogen on the anode is converted to a proton and then moves together with water onto the cathode. The hydrogen reacts with oxygen on the cathode to produce water. When such a fuel cell is to be operated, therefore, the fuel cell must have a structure in which sufficient supply of the reactant gasses, discharge of the produced water, and output of the current are enabled. Various kinds of such structures have been proposed.
FIG. 15 is a sectional view showing a basic configuration of a unit cell constituting a fuel cell. In the figure, 1 denotes a first separator plate which is electrically conductive and on which groove-like oxidant flow paths 10 are engraved, 2 denotes a second separator plate which is electrically conductive and on which groove-like fuel flow paths 11 are engraved, 3 denotes a cathode, 4 denotes an anode, and 5 denotes an electrolyte membrane in which, for example, a proton-conductive solid polymer is used. As described above, various structures for a separator plate of a fuel cell having a thus configured section have been proposed. FIG. 16 is a top view of a separator plate which is similar to that disclosed in, for example, Japanese Patent Publication (Kokai) No. HEI3-205763.
In the figure, 10 denotes plural oxidant flow paths which are formed in one face of the first separator plate 1 in a meandering form, or on the side of the face contacted with the cathode 3, in order to supply an oxidant gas to the cathode 3. For example, oxygen gas or air is used as the oxidant gas. In FIG. 16, the oxidant flow paths 10 elongate in a meandering form so as to have a large length, whereby the gas flow rate is increased, and are made shallow so as to expedite diffusion of gasses required for reaction. As a result, water produced in the cathode 3 is efficiently discharged.
The reference numeral 31 denotes the principal face of the first separator plate 1, 32 denotes an electrode support portion which supports the cathode 3, 24 denotes an air supply port which is formed in the first separator plate 1 and through which air is supplied, and 25 denotes an air discharge port through which air is discharged. Although not shown in the figure, also the second separator plate 2 has a similar structure and is provided with meandering fuel gas flow paths 11 in place of the oxidant flow paths 10. Each oxidant flow path 10 is configured by a space defined by the groove of the first separator plate 1 and the cathode 3, and each fuel gas flow path (also referred to as the fuel flow path) 11 is configured by a space defined by the groove of the second separator plate 2 and the anode 4. For example, hydrogen gas is used as the fuel gas which flows through the fuel flow path 11.
Next, the operation will be described. The oxygen gas which is fed through the air supply port 24 of the first separator plate 1 is supplied to the cathode 3 through the plural oxidant flow paths 10 which elongate in parallel. By contrast, the hydrogen gas (fuel gas) is supplied to the anode 4 through the fuel gas flow paths 11. At this time, the cathode 3 and the anode 4 are electrically connected outside the fuel cell to each other via a load. Therefore, reaction (2) occurs on the side of the cathode 3, and unreacted gas and water are discharged to the air discharge port 25 through the oxidant flow paths 10. On the other hand, reaction (1) occurs on the side of the anode 4, and unreacted gas is discharged through the fuel gas flow paths 11 and a fuel discharged port (not shown). Electrons which are obtained as a result of this reaction flow through the electrodes 3 and 4, the electrode support portion 21, and the first and second separator plates 1 and 2.
The separator plates and the electrodes are stacked and then brought into contact under pressure by applying a load. In order to cause the fuel cell to efficiently operate, the reaction distribution in the face of one separator must be made as uniform as possible. Therefore, the load which is applied to the cell face must be made uniform. FIG. 17 shows a technique disclosed in U.S. Pat. No. 5,484,666. In the figure, 100 denotes a stack of single cells such as shown in FIG. 15, 35 denotes through holes which are formed in the cell stack 100, 36 denotes bolts which are passed through the through holes 35, 37 denotes disc springs which are attached to ends of the bolts 36, 39 denotes end plates which are attached to ends of the cell stack 100, and 50 denotes cavities which are formed in the end plates 39 so as to accommodate the disc springs 37.
In FIG. 17, many (in the figure, four) bolts 36 are disposed ir the face of the cell stack 100 so as to make the load uniform. However, the formation of many holes in the electrode faces complicates the gas flow paths, and increases the number of gas sealing portions, thereby increasing area loss.
Each of the plural oxidant flow paths 10 and fuel gas flow paths 11 is configured as a single flow path from the inlet to the outlet or has no ramification such as a branch or a junction. When even one portion in the midpoint of the flow path is clogged, therefore, the gas is not newly supplied in the whole of the flow path or in the entire range from the inlet to the outlet, so that the function of the whole of the flow path is disabled. As a result, the power generation capacity is naturally lowered.
Each of the cells is structured so as to independently receive supplies of a fuel gas and an oxidant gas. When the function of the flow paths is partly lowered in even one of the stacked cells so that the fuel is deficient in the one cell, a reaction indicated by the following formula occurs in the electrode: EQU C+2H.sub.2 O.fwdarw.CO.sub.2 +4H++4e- (3)
As a result, carbon in components of the fuel cell, such as the electrodes and the separators is corroded and the electrodes and the like suffer fatal damage, with the result that the power generation capacity of the whole fuel cell is extremely reduced.
In a fuel cell of the conventional art, as described above, many bolts are used in order to make uniform the face pressure. Therefore, the effective area of each separator is reduced, thereby producing a problem in that the power generation efficiency is lowered.
Each cell is structured so as to independently receive supplies of a fuel gas and an oxidant gas. When fuel gas is deficient in even one cell, therefore, carbon is corroded causing a fear that cells will be fatally damaged.
Each of oxidant flow paths and fuel gas flow paths is configured as a single flow path from the inlet to the outlet or has no ramification such as a branch or a junction. When even one portion in the midpoint of such a flow path is clogged, therefore, the gas is not newly supplied in the whole of the flow path or in the entire range from the inlet to the outlet, thereby producing a problem in that the function of the whole of the flow path is disabled.